This R21 proposal is submitted in response to RFA-RM-10-015 the Economics of Prevention. Our proposed randomized controlled trial (RCT) is designed to determine whether well-informed patients can persuade doctors to reduce the number of inappropriate screening tests and increase the number of appropriate preventive screening tests that are ordered during routine medical visits. Up to 93% of doctors self-report ordering unnecessary tests. Unnecessary tests are those that, on balance, appear to produce more harm than good. Harm to the patient can arise directly from the test (e.g., infection, bleeding, or exposure to radiation) or from false positive test results. False positive test results often produce psychological distress for the patient by leading to further invasive testing. Inappropriate testing can also cause indirect harm by needlessly increasing health system costs, and has been estimated to cost between $50 billion and $650 billion annually. Medical care costs outstrip consumer inflation, and cost increases threaten the long-term viability of the Patient Protection and Affordable Care Act (PPACA). A wide range of studies has shown that well-informed patients can influence physician behavior. We have the unique opportunity to randomly assign approximately 1,128 predominately young and healthy female employees at a large Silicon Valley technology firm to either receive a 3patient activation intervention plus a health message or to receive the message alone. The patient activation intervention entails the use of an innovative education tool to inform patients regarding the appropriate clinical practice for their age, gender, and general risk factors for disease. It also provides guidance on how to communicate appropriate test ordering behavior with their physician. Follow-up will be monitored automatically via billing systems and web-based self-administered surveys, allowing us to conduct an RCT at very low cost. We will then use these data to conduct a cost- effectiveness analysis of the intervention. We will also, for the first time, use RCT data to explore psychological distress induced by false positive mammograms among women under 50 years of age who have no risk factors for breast cancer. Patient activation is a politically neutral and innovative means of increasing the efficiency of preventive healthcare delivery in the U.S. It has the potential to optimize preventive service delivery within PPACA. But its effectiveness will be greatest if it changes the culture of medicine by spreading within patient and provider social networks. Therefore, in addition, this study will also serve to pilot an R01 to track behavior change as an intervention spreads through the 12,500 employees within the social network of a large Silicon Valley technology firm and their provider networks. Information gathered from these programs-in the form of subject interaction tracking via Global Positioning Service-will eventually inform an agent-based systems model examining strategies for improving the efficiency of the US health care system.